There are 148 color keywords defined in CSS 3. However, there are only 139
individual colors. This is because there are a pair of color keywords for each of
the gray colors — one spelt "gray" and the other spelt "grey". Also,
fuchsia and magenta is the same color, and so is aqua and cyan.

Internet Explorer 6 and 7 ignore the following color keywords:
lightgray, darkgrey, grey, dimgrey,
lightslategrey, slategrey, darkslategrey.
Therefore, the keywords lightgrey, darkgray,
gray, ddimgray,
lightslategray,
slategray, darkslategray can be used instead because they refer to the same colors

System Color Keywords

The system color keywords refer to the colors used by the
operating system in its GUI. These keywords make it
possible to create a web page with an interface that mimics the viewer's
operating system GUI. CSS 2.1 specifies the following system color
keywords (source:
http://www.w3.org/TR/CSS2/ui.html#system-colors).

Hexadecimal Notation

A hexadecimal notation color value consists of the pound sign (#) followed by six or three hexadecimal digits - example: #344da4. The first two digits (from the left) represent red, the next two digits represent green and the last two represent blue.

Since each two digit pair is in hexadecimal, the maximum value is ff or 255 in decimal. Thus, each of the three primary colors - red, green, blue are represented by 256 different levels.

For example, #344da4 represents a red level of 34 hexadecimal or 52 decimal, a green level of 4d hexadecimal or 77 decimal and a blue level of a4 hexadecimal or 164 decimal. A level of 255 represents the strongest level of a particular color and a level of 0 represents the weakest level - complete absence of the color.

Numbers can be converted from hexadecimal to decimal and vice versa using a calculator.

In the above example, the color value can be expanded to #AADD99 which
is equivalent to rgb(170,221,153).

Functional RGB Notation

The Functional RGB Notation consists of a decimal number triplet of either percentages or integers. The triplet represents the three primary colors red, green and blue.

Decimal Functional RGB Notation

In this notation the rgb triplet is specified as three comma separated decimal numbers enclosed in parentheses. The range of values is 0 to 255. Here is an example of the functional rgb notation using a decimal number triplet: rgb(156,45,56). In this example the level of red is 156, green is 45 and blue is 56.

Percentage Functional RGB Notation

In this notation the rgb triplet is specified as three comma separated decimal percentages enclosed in parentheses. The range of values is 0% to 100%, decimal values (example 10.5%) are allowed. Any percentage greater than 100% is considered equal to 100%. Example: (350% is considered 100%). The color in the previous decimal functional rgb notation can be converted to percentage function rgb notation using the formula:

(primary color value ÷ 255) × 100

Only one decimal place is necessary since the rounded-off computed value can
represent all the numbers from 0 to 255.